I'm using a Mac, and my outgoing mail is being blocked and I get the message "Error: "554 5.7.9: Message not accepted for policy reasons" when sending email to Yahoo"

I'm using a Mac, and my outgoing mail is being blocked and I get the message "Error: "554 5.7.9: Message not accepted for policy reasons" when sending email to Yahoo". How do I fix this? I've been using the same email programs (gmail and aol) for years without a problem, now this bunch of bs pops up.